FIG. 1 is a sectional view illustrating a prior art long-wave semiconductor laser device. In FIG. 1, reference numeral 1 designates a p type InP substrate 400 .mu.m thick. A p type InP cladding layer 2 having a dopant concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 1 .mu.m is disposed on the p type InP substrate 1. An MQW (multiquantum well) active layer 3 is disposed on the p type InP cladding layer 2. The structure of the active layer 3 will be described later. An n type cladding layer 4 having a dopant concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 0.5.mu.m is disposed on the MQW active layer 3. The p type cladding layer 2, the MQW active layer 3, and the n type cladding layer 4 have a mesa shape. A p type InP mesa embedding layer 5 having a dopant concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 1 .mu.m is disposed on the p type cladding layer 2, contacting the opposite sides of the mesa. An n type InP current blocking layer 6 and a p type InP current blocking layer 7 are successively disposed on the mesa embedding layer 5. The n type InP current blocking layer 6 and the p type InP current blocking layer 7 have dopant concentrations of 5.times.10.sup.18 cm.sup.-3 and 1.times.10.sup.18 cm.sup.-3, respectively, and a thickness of 1 .mu.m. A p side electrode 8 comprising Ti/Pt/Au is disposed on the rear surface of the substrate 1, and an n side electrode 9 comprising Au/Ge/Ni/Au is disposed on the n type cladding layer 4. An insulating film 10 comprising SiO.sub.2 is disposed over the front surface of the structure where the n side electrode 9 is absent.
FIG. 2 is a sectional view showing a part of the laser structure of FIG. 1 in the vicinity of the MQW active layer 3. In FIG. 2, reference numeral 11 designates an InGaAsP light confining layer, numeral 12 designates an InGaAs or InGaAsP well layer, and numeral 13 designates an InGaAsP barrier layer. FIG. 3 is an energy band diagram of the structure shown in FIG. 2.
When current flows between the p side electrode 8 and the n side electrode 9, holes and electrons are injected into the active layer 3 from the p type cladding layer 2 and the n type cladding layer 4, respectively. The injected holes and electrons recombine in the active layer 3, whereby laser oscillation occurs. The holes and the electrons pass through the InGaAsP light confining layer 11 and the InGaAsP barrier layer 13 and are stored in the InGaAs or InGaAsP well layer 12. Then, the holes and the electrons recombine to produce light.
However, the prior art semiconductor laser device has the following drawbacks. When carriers are injected into a plurality of wells and the number of the wells is large, the distribution of the carriers (especially holes) in the wells is uneven, causing an increase in the carrier concentration at the side of the p type cladding layer 2. As a result, light is also unevenly distributed at the side of the p type cladding layer 2, whereby the emission efficiency is lowered. In addition, the distribution of the carriers between the wells causes a difference in pseudo-Fermi level between the wells, so that the gain spectrum is broadened. Consequently, the high speed modulation characteristic of the laser device is limited.
As described above, in the prior art semiconductor laser device having an MQW active layer, since compositions and thicknesses of the barrier layers in the MQW structure are uniform, injection of carriers becomes uneven, so that the emission efficiency is low. As a result, the high speed modulation characteristic is not preferable, and the modulation bandwidth is narrow.